dotphoto identifies projects by the first photo in the project, so, if you try to make multiple cards, ornaments or other products that share the same main image, the dotphoto cart will only accepts one project or one item.
This is the workaround: if the main photos is even one pixel different, dotphoto sees it as a different image. Use the dotphoto editor to modify the image slightly. Tap on the photo, and tap the pencil icon in the upper left to launch the editor. Tap Image, choose Image Size, change the width by 1 pixel, and tap OK.
Then change the dimensions by 1 pixel, and File | Save. You will have a second version of the same photo for the second project.